diff --git a/submission_thesis/CH2_FMEA/copy.tex b/submission_thesis/CH2_FMEA/copy.tex index a8dc861..cbfb964 100644 --- a/submission_thesis/CH2_FMEA/copy.tex +++ b/submission_thesis/CH2_FMEA/copy.tex @@ -15,13 +15,14 @@ on the behaviour and safety of the system." %\tableofcontents[currentsection] -FMEA is a broad term, and can mean anything from an informal check on how +FMEA is a broad term; it could mean anything from an informal check on how how failures could affect some equipment in an initial brain-storming session -in product design to formal submissions as part of safety critical certification -procedures -This chapter describes the basic concepts, uses a simple example to -demonstrate an FMEA stage and then explores some concepts with which we can evaluate - the effectiveness of FMEA. +in product design, to formal submissions as part of safety critical certification. +% +This chapter describes basic concepts of FMEA, uses a simple example to +demonstrate a single FMEA analysis stage, describes the four main variants of FMEA in use today +and explores some concepts with which we can discuss and evaluate +the effectiveness of FMEA. % \subsection{FMEA} @@ -46,8 +47,8 @@ demonstrate an FMEA stage and then explores some concepts with which we can eval % % \item Analysis % % \end{itemize} - -\subsection{FMEA basic concept} +\clearpage +\paragraph{FMEA basic concept.} \begin{itemize} @@ -62,7 +63,7 @@ demonstrate an FMEA stage and then explores some concepts with which we can eval FMEA is a procedure based on the low level components of a system, and an example analysis will serve to demonstrate it in practise. - \subsection{ FMEA Example: Milli-volt reader} + \paragraph{ FMEA Example: Milli-volt reader} Example: Let us consider a system, in this case a milli-volt reader, consisting of instrumentation amplifiers connected to a micro-processor that reports its readings via RS-232. @@ -89,7 +90,7 @@ For the sake of example let us choose resistor R1 in the OP-AMP gain circuitry. - \subsection{FMEA Example: Milli-volt reader} + \paragraph{FMEA Example: Milli-volt reader} % \begin{figure} % \centering % \includegraphics[width=80pt]{./mvamp.png} @@ -105,14 +106,21 @@ For the sake of example let us choose resistor R1 in the OP-AMP gain circuitry. - +The analysis above has given us a result for one failure scenario i.e. +for one component failure mode. +A complete FMEA report would have to contain an entry +for each failure mode of all the components in the system under investigation. +% Note here that we have had to look at the failure~mode in relation to the entire circuit. We have used intuition to determine the probable effect of this failure mode. -We have not examined this failure mode +For instance we have assumed that the resistor R1 going SHORT +will not affect the ADC, the Microprocessor or the UART. +% +To put this in more general terms, have not examined this failure mode against every other component in the system. -Perhaps we should.... this would be a more rigorous and complete +Perhaps we should: this would be a more rigorous and complete approach in looking for system failures. @@ -135,20 +143,18 @@ are thus very reliable too. Reliable field data on failures will, therefore be s Should we wish to prove a continuous demand system for say ${10}^{-7}$ failures\footnote{${10}^{-7}$ failures per hour of operation is the threshold for S.I.L. 3 reliability~\cite{en61508}.} per hour of operation, even with 1000 correctly monitored units in the field -we could only expect one failure per ten thousand hours (a little over one a year) to fail. -It would be impractical to get statistically significant data for equipment +we could only expect one failure per ten thousand hours (a little over one a year). +It would be utterly impractical to get statistically significant data for equipment at these reliability levels. However, we can use FMEA (more specifically the FMEDA variant, see section~\ref{sec:FMEDA}), working from known component failure rates, to obtain statistical estimates of the equipment reliability. -\subsection{Rigorous FMEA --- State Explosion} +\subsection{Rigorous FMEA --- State Explosion Problem} -FMEA cannot consider---for practical reasons---a rigorous approach. -It must be applied by experts in the system under investigation -to be a meaningful analysis. - \paragraph{Rigorous Single Failure FMEA} + +\paragraph{Rigorous Single Failure FMEA} FMEA for a safety critical certification~\cite{en298,en61508} will have to be applied to all known failure modes of all components within a system. @@ -191,7 +197,14 @@ For our theoretical 100 components with 3 failure modes each example, this is $100*99*98*3=2,910,600$ failure mode scenarios. - +\paragraph{Reliance of experts for meaningful FMEA Analysis.} +FMEA cannot consider---for practical reasons---a rigorous approach. +We define rigorous FMEA as examining the effect of every component failure mode +against the remaining components in the system under investigation. +% +Because we cannot perform rigorous FMEA, +we rely on experts in the system under investigation +to perform a meaningful FMEA analysis. @@ -214,7 +227,7 @@ $100*99*98*3=2,910,600$ failure mode scenarios. \section{PFMEA - Production FMEA : 1940's to present} - \subsection{PFMEA} + Production FMEA (or PFMEA), is FMEA used to prioritise, in terms of cost, problems to be addressed in product production. @@ -279,7 +292,6 @@ will return most cost benefit. - \subsection{PFMEA Example: Ford Pinto: 1975} \begin{figure}[h] \centering @@ -293,8 +305,6 @@ will return most cost benefit. - - \subsection{PFMEA Example: Ford Pinto: 1975} \begin{table}[ht] \caption{FMEA Calculations} % title of Table @@ -315,7 +325,7 @@ will return most cost benefit. - http://www.youtube.com/watch?v=rcNeorjXMrE +% don't think this is relevant for the thesis: http://www.youtube.com/watch?v=rcNeorjXMrE